The present invention relates to a dielectric constant detection apparatus for fuel which detects a dielectric constant of fuel supplied to a burner or the like in a non-contact state to thereby detect the property of the fuel. More particularly, it relates to such apparatus to measure the alcohol content of an alcohol blended fuel used for the engine of an automobile.
Fuel in which alcohol is blended in gasoline has been used for automobiles to thereby reduce consumption of oil and to reduce air pollution due to exhaust gas from automobiles in the U.S.A. and European countries. When the alcohol blended fuel is used for an automobile engine which is adjusted regarding an air fuel ratio of gasoline, the air-fuel ratio becomes lean because the theoretical air-fuel ratio of alcohol is smaller than that of the gasoline, whereby it is difficult to obtain stable driving. Accordingly, the alcohol content of the alcohol blended fuel is detected to thereby adjust the air fuel ratio, ignition timing and so on. Thus, adjustment is made on the basis of a detected value.
In order to detect the alcohol content, there have been proposed a system of detecting the dielectric constant of alcohol blended fuel and a system of detecting the refractive index of the fuel. As a conventional apparatus used for the system of detecting the dielectric constant of the alcohol blended fuel, such one as disclosed in Japanese Examined Patent Publication No. 31734/1988 was utilized to detect the dielectric constant of the fuel in a non-contact state. Now, description will be made with reference to FIGS. 3, 11 and 12 as to a case that the detection apparatus is used for detecting an alcohol content in alcohol blended fuel.
FIG. 11 is a diagram showing the conventional detection apparatus for detecting a dielectric constant of fuel, wherein reference numeral 1 designates an insulation tube made of an insulation material such as ceramics, oil-resisting plastics or the like and in which a fuel passage 2 is formed, numeral 3 designates an electrically conductive electrode wound in a ring form on a part of the insulation tube 1, and numeral 4 designates a monolayer-wound coil wound on the insulation tube 1 so as to be apart from the electrode 3 by a predetermined distance. A sensor member is formed of the above-mentioned elements 1 through 4. Reference numeral 20 designates a fixed frequency oscillator to apply a voltage signal having a fixed frequency to the electrode 3 through an amplifier 15. The coil 4 has a terminal grounded and the other terminal through which a signal is outputted as an output V.sub.out through a high-pass filter 17, a full wave rectifier 18 and the amplifier 15.
FIG. 3 is a diagram showing the frequency characteristics of the sensor member in which there are shown voltages induced in the coil 4 and the change of the phases of the induced voltage signals by the induced signals of the electrode 3 in a case that gasoline (line a) and methanol (line b) are used as fuel, and the frequency is changed while a voltage applied to the electrode 3 is constant. FIG. 12 is a diagram showing the output characteristics of the conventional apparatus in which an output V.sub.out to a methanol content of fuel in the conventional apparatus is shown in a case that a signal having a fixed frequency of as shown in FIG. 3 is supplied to the electrode 3 from the fixed frequency oscillator 20.
The operation of the conventional apparatus will be explained.
In FIG. 11, when a frequency applied to the electrode 3 is changed, a voltage induced in the coil 4 shows the maximum value at a different frequency in a case of a different dielectric constant of fuel. It is because LC (inductance-capacitance) resonance is resulted due to a capacitance Cf corresponding to a dielectric constant e of fuel between the electrode 3 and the coil 4 and the self-inductance L of the coil 4, whereby the induced voltage of the coil becomes maximum at the resonance frequency. The resonance frequency f is expressed by the following formula: EQU f=k/.sqroot.{L.times.(Cf+Cs)}=k/.sqroot./(a+b.times..epsilon.)(1)
where Cs represents the capacitance of a monolayer-wound coil and k, a and b are respectively constants determined by the shape of the sensor member. Since the resonance frequency f depends on the dielectric constant .epsilon. of the fuel as shown in the above-mentioned formula, the resonance frequency decreases as the dielectric constant .epsilon. becomes large. For instance, in the measurements of the resonance frequency with use of a sensor member having a specified shape, it was found that the resonance frequency fm was about 5 MHz for methanol having a dielectric constant .epsilon. of 33 and the resonance frequency fg was about 5.7 MHz for gasoline having a dielectric constant e of 2. In view of the above, when methanol blended gasoline is introduced in the fuel passage 2 in FIG. 11 and a signal having a frequency of which is slightly higher than the resonance frequency fm is generated from the fixed frequency oscillator 20 to thereby excite the electrode 3 at a fixed voltage through the amplifier 15, only an a.c. component in the voltage signal induced in the coil 4 is extracted by the high-pass filter 17. The a.c. component is subjected to full wave rectification in the full wave rectifier 18 and the amplitude of the alternating current is detected. The detected amplitude is adjusted to have a predetermined voltage range by the amplifier 15 and the adjusted signal is outputted. Thus, the induced voltage having the frequency of becomes large as the methanol content is large (FIG. 3). Accordingly, the output V.sub.out is substantially proportional to the methanol content as shown in FIG. 12.
The conventional apparatus, however, had the disadvantage as follows. When impurities having a high electric conductivity, e.g. metal ions resulted from broken pieces of a fuel distribution system, rust or the like are mixed in fuel, or a slight amount of an ion series additive for fuel, rain water or the like invades in it, the output V.sub.out shows a remarkable change even though the dielectric constant .epsilon., i.e. the methanol content, is not substantially changed, as indicated by a broken line b' in FIG. 3. This is because the Q factor of LC resonance decreases due to an increased electric conductivity of fuel, and the induced voltage is greatly reduced at the same frequency fo. Further, when temperature and humidity in the atmosphere around the sensor member change, the insulation resistance between the electrode 3 and the coil 4 is changed, and accordingly, there causes a change of the output. Because of a change of the electric conductivity between the electrode 3 and the coil 4 due to a slight amount of impurities or a change in the atmosphere around the sensor member, it is difficult to detect correctly the methanol content.